Cesian Bazzite and Thortveitite from Cuasso Al Monte, Varese, Italy: a Comparison with the Material from Baveno, and Inferred Origin

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The Canadian Mineralogist Vol. 38, pp. 1409-1418 (2000)

CESIAN BAZZITE AND THORTVEITITE FROM CUASSO AL MONTE, VARESE, ITALY: A COMPARISON WITH THE MATERIAL FROM BAVENO, AND INFERRED ORIGIN

CARLO MARIA GRAMACCIOLI Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy

VALERIA DIELLA Centro CNR per la Geodinamica alpina e quaternaria, Via Botticelli 23, I-20133 Milano, Italy

FRANCESCO DEMARTIN Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, Via Venezian 21, I-20133 Milano, Italy

PAOLO ORLANDI Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 53, I-56100 Pisa, Italy

ITALO CAMPOSTRINI Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy

ABSTRACT

The occurrence of thortveitite, Sc2Si2O7, and bazzite, Be3Sc2Si6O18, in the miarolitic cavities of the granophyre of Cuasso al Monte, in Varese, Italy, is reported for the first time, together with additional data for the corresponding species from the granite of Baveno. Bazzite from both these localities is Mg-poor and contains notable amounts of cesium (up to 2.3 wt% Cs2O), which is not encountered in the corresponding samples from Alpine fissures, and which has been overlooked in the type material (from Baveno). These discoveries confirm the similarity between the Baveno and Cuasso al Monte occurrences, and suggest the possibility that scandium minerals are more widespread in granitic rocks than is inferred at present. The composition and paragenesis of these Sc-rich species indicate the importance of complexes in enhancing the different geochemical behavior of scandium with respect to the REE, and that of equilibria with species such as feldspars, where aluminum cannot be replaced by transition elements.

Keywords: scandium, thortveitite, bazzite, Baveno, Cuasso al Monte, Italy.

SOMMAIRE

Nous signalons pour la première fois la présence de thortvéitite, Sc2Si2O7, et de bazzite, Be3Sc2Si6O18, dans les miaroles d’un granophyre à Cuasso al Monte, dans la province de Varese, en Italie, et nous fournissons des observations nouvelles sur les mêmes espèces dans le granite de Baveno. A chaque endroit, la bazzite a une faible teneur en Mg et est enrichie en césium (jusqu’à 2.3% de Cs2O, en poids), ce qui n’avait pas été signalé dans la bazzite de la localité-type (Baveno). Ce ne sont pas les caractéristiques d’échantillons de ces espèces récupérés des fentes alpines. Ces nouvelles observations confirment la ressemblance de Baveno et de Cuasso al Monte, et fait penser que les minéraux de scandium pourraient bien être plus répandus dans les roches granitiques que l’on ne le soupçonne présentement. La composition et la paragenèse des espèces à Sc montre l’importance de complexes aqueux à distinguer le comportement géochimique du scandium par rapport aux terres rares, et d’équilibres ioniques avec des espèces telles que les feldspaths, dans lesquels l’aluminium ne peut pas être remplacé par des éléments de transition.

Mots-clés: scandium, thortvéitite, bazzite, Baveno, Cuasso al Monte, Italie.

¤ E-mail address: [email protected]

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INTRODUCTION We thus decided to carry out a more detailed investigation of these minerals from Cuasso al Monte. At the The discovery of bazzite, Be3Sc2Si6O18, in miarolitic same time, one cotype specimen of bazzite in good crys- cavities of the Baveno granite, in the Lago Maggiore tals and samples of thortveitite from Baveno had be- area of Italy, by Artini (1915) followed closely come available to us, thereby providing the possibility Eberhard’s (1908, 1910) discovery of thortveitite in the of obtaining improved crystallographic and composi- Iveland region in Norway as the first known mineral in tional data on these minerals from both localities. which scandium is present as an essential constituent. The original find at Baveno remained the only one there X-RAY DATA until about 1980, although several occurrences of bazzite have been discovered in Alpine fissures since Bazzite 1939 (see Hänni 1980 for references). At the same time, additional scandium-rich species have been observed at A fragment of a tiny blue, bazzite-like crystal from Baveno: for instance, thortveitite Sc2Si2O7 occurs in Cuasso al Monte was examined using a Nonius CADÐ4 very unusual specimens as tiny greyish or deep blue single-crystal X-ray diffractometer. Although the poor crystals, in miarolitic cavities (Orlandi 1990). Other quality of the crystal prevented the possibility of refin- important discoveries are the new minerals cascandite ing the structure, the data were sufficient for deducing CaScSi3O8(OH) (Mellini & Merlino 1982, Mellini et al. the Laue symmetry 6/mmm and the unit-cell parameters; 1982), jervisite NaScSi2O6 (Mellini et al. 1982), and these results are reported in Table 1. On this basis, the 2+ scandiobabingtonite Ca2(Fe ,Mn)ScSi5O14(OH) identity of the mineral from Cuasso al Monte with (Orlandi et al. 1998). For a detailed history of all such bazzite is confirmed; final proof was provided by quan- discoveries, see Gramaccioli et al. (1998). titative electron-microprobe analysis (see below). The In recent years, the scandium-rich species and their large unit-cell dimensions constitute a peculiarity of paragenesis have received attention. Such an increase bazzite from this new locality; the high value of c may of interest is witnessed by a number of recent discover- relate to the presence of notable amounts of divalent ies of scandium minerals (besides the literature concern- ions, such as Fe2+, substituting for Sc3+, but the high ing Baveno, see also Bergst¿l & Juve 1988, Juve & value of a cannot easily be explained. Bergst¿l 1990, Armbruster et al. 1995, Liferovich et al. 1997, 1998, Bernhard et al. 1998, Raade & Erambert 1999); furthermore, the geochemical behavior of Sc, which is distinct with respect to the rare-earth elements (REE), has been shown not to be due only to the differences in ionic radii. Recent observations have empha- sized the importance of complexes of Sc in the depositing solutions (Gramaccioli et al. 1999a, b, 2000). On considering the granophyre from Cuasso al Monte, Lombardy, Italy, and from the nearby locality of Carona in Canton Ticino, Switzerland, and its relationships with the Baveno granite (Bakos et al. 1990, Boriani et al. 1992), scandium minerals were expected to occur here for some time. However, in 1992 an inter- esting specimen was noticed by Mr. R. Appiani in a quarry (Cava Laghetto) at Cuasso al Monte: it consists of a group of very tiny (maximum length 0.1 mm) greyish blue hexagonal prismatic crystals with flat termina- tions and similar to bazzite from Baveno, perched upon “orthoclase” (microcline) in a miarolitic cavity, associated with quartz, albite, and a black biotite-like mineral. Furthermore, in 1994, another mineral collector, Mr. F. Anderbegani, submitted to our attention a certain number of specimens of another mineral he had found in the same quarry, showing very tiny groups of whitish aggregates of crystals. These aggregates are perched upon “orthoclase”, are associated with tourmaline, fluorite, quartz and albite, and are similar to greyish thortveitite from Baveno. A preliminary energy-dispersion analysis showed the presence of notable amounts of scandium in both cases.

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The cotype specimen of bazzite from Baveno exam- persion spectrometer; the results are reported in Tables ined is a part of the old Bazzi collection, which is now 5 and 6. The system was operated at an accelerating preserved in the mineralogical collection of Diparti- voltage of 15 kV, a sample current on brass of 10 nA, mento di Scienze della Terra, University of Milan. From and a counting time of 10 or 20 seconds on the peaks, this sample, a very tiny fragment of a crystal was se- and 5 or 10 seconds on the backgrounds. Kaersutite (for lected, and very good data were obtained using a CADÐ Mg, Na,Ti, K), spessartine (for Fe and Mn), omphacite 4 single-crystal diffractometer, permitting accurate (for Si and Ca), pollucite (for Cs), and zircon (for Zr) refinement of the structure (Demartin et al. 2000). The unit-cell values (Table 1) are in line with recent values obtained for bazzite from other localities, but are notably different with respect to the previous set determined by Peyronel (1956) on type (or cotype) material as well. Such a difference is almost certainly due to the poor accuracy of the older measurements, especially reflected in the anomalously small value of c.

Thortveitite

For the other scandium-bearing mineral from Cuasso al Monte and all thortveitite specimens from Baveno, no single crystals providing reliable X-ray data could be found. The X-ray powder-diffraction patterns of two different samples of this mineral from Cuasso al Monte, obtained using a Gandolfi camera, are reported in Table 2, together with those of a sample from Norway and of pure synthetic thortveitite for comparison. The corresponding patterns obtained from four different samples of thortveitite from Baveno are reported in Table 3, and the unit-cell parameters, obtained from a least-squares fit of these powder data, are reported in Table 4. They do not differ significantly, and agree satisfactorily with the corresponding data on “classic” occurrences in Nor- way and Madagascar (Bianchi et al. 1988), as well with those for a pure synthetic sample of Sc2Si2O7. The composition of thortveitite is variable; in some cases, the mineral is almost pure, and in others it contains notable quantities of yttrium, zirconium, and the heaviest REE. As was shown by several authors (Smolin & Shepelev 1970, Voloshin et al. 1983, 1985, Bianchi et al. 1988, Gramaccioli et al. 1999b, Foord et al. 1993), the unit-cell parameters a and b increase appreciably on replacing Sc by Y and the REE, whereas c and ␤ remain almost constant. By following only this criterion, and considering the data reported in Table 4, the first specimen from Cuasso al Monte and the blue specimens from Baveno represent a Y- (and REE-)poor variety of thortveitite (see below). For the second specimen from Cuasso al Monte, the data were insufficient to deduce the unit-cell parameters, and the X-ray powder-diffraction pattern (Gandolfi) only supports the identification.

CHEMICAL COMPOSITION

In addition to a series of preliminary EDS analyses, quantitative chemical analyses were performed on pol- ished samples using an Applied Research Laboratories electron microprobe fitted with six wavelength-dispersion spectrometers and a Tracor Northern energy-dis-

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were employed as standards; with respect to Sc,Y, Zr, presence was first noticed on examining EDS spectra of Hf, and the REE, the lines, the standards used and the type and cotype specimens (Gramaccioli et al. 1998). procedures followed to account for interferences are Bertolani (1948) used spectrographic analysis to described in detail in Bianchi et al. (1988), Demartin et detect the presence of Y and REE in type (or cotype) al. (1991) and Mannucci et al. (1986). The results were specimens. These elements have not been detected with processed for matrix effects using a modified version of our electron microprobe, as with material from other the MAGIC IV program. localities (see, for instance, Hänni 1980); such a discrep- ancy is certainly due to the higher sensitivity of a spec- Bazzite trographic analysis, which, on the other hand, should have revealed the presence of cesium; the early volatil- Our results are reported in Table 5, together with ization of Cs in the emitting source with respect to the significant data from other localities taken from the lit- other elements considered is a probable reason for this erature. There is a noticeable compositional difference error. between the samples from Alpine fissures and those As in beryl (Hänni 1980, Aurisicchio et al. 1988, from the other locations; the former contain important Artioli et al. 1993), the charge balance in bazzite to ac- amounts of magnesium, and the latter contain instead count for the presence of divalent ions like Mg or Fe2+ non-negligible amounts of heavy alkalis, especially ce- in the 4(c) Sc (or Al) site can be achieved mainly owing sium. This element was overlooked in previous investi- to the presence of alkali metal ions (Na+ and Cs+) in the gations of the type material (Bertolani 1948); its channels; a similar observation could also be drawn for

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results of the crystal-structure refinement; however, such procedures may not be entirely correct. The actual contents of beryllium were determined by Chystyakova et al. (1966) for bazzite from Kazakhstan, by Juve & Bergst¿l (1990) for bazzite from the Heftetjern pegmatite, T¿rdal, Norway, and by Della Ventura et al. (2000) for type stoppaniite. In these instances, an excess of Be

the newly discovered species stoppaniite, the Fe-rich member of the beryl group (Ferraris et al. 1998, Della Ventura et al. 2000). If all the iron in the specimen from Baveno is assumed to occupy the 4(c) sites in the form of Fe2+, the charge of the alkali metals in the channels would be balanced almost exactly on replacing the Sc3+ ions by the 2+ (and 4+) ions present in the structure. The same, however, does not happen for the specimen from Cuasso al Monte, and on these grounds, the presence of some Fe3+ instead of Fe2+ might be assumed as well (Aurisicchio et al. 1988, Artioli et al. 1993). Similarly, bazzite from Alpine fissures contains more magnesium than necessary to balance the charges in this way, even without considering a possible role of iron. Therefore, in agreement with Armbruster et al. (1995), the presence of some 2+ ions in the channels cannot be ruled out, at least in some cases. Furthermore, other ways of balancing the charges cannot be excluded: these possibilities are the presence of OHÐ groups in the structure, or the partial substitution of Si4+ by Be2+ (see below) or even of Be2+ by Li+. Therefore, in view of these possibilities and in lack of additional information, no attempt has been made to derive the Fe2+:Fe3+ ratio. The low analytical total in some cases (including bazzite from Cuasso al Monte) is almost certainly due to ne- glect of the presence of H2O, which might also be present in the channels. An adequate crystal-chemical characterization of all the minerals in the beryl group would imply an accurate determination of the contents of the lightest elements, such as Be, Li, or H, on a significant number of specimens. In view of the lack of such information, most data concerning Be and H reported in Table 5 have been derived from simple assumptions (e.g., by imposing a value of 0.5 to the atomic ratio Be/Si), or using the

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was interpreted as evidence for partial replacement of The geochemical behavior of scandium is peculiar and Si by Be, corresponding to the proportions Si5.93Be0.07, apparently contradictory: in igneous rocks, Sc is usu- Si5.7Be0.3, and Si5.53Be0.44 at the Si site, respectively; for ally concentrated in ferromagnesian minerals, and for stoppaniite, such a replacement is in agreement with the this reason it usually prefers mafic assemblages, but crystallographic results obtained by Ferraris et al. notable exceptions are found in some carbonatites, (1998). This possibility was considered in our crystal- where the accessory minerals are instead the major hosts structure refinement of the sample from Baveno and of of Sc (Eby 1973). another sample from T¿rdal, Norway; however, no defi- Other important concentrations of this element oc- nite conclusions in this respect could be achieved using cur in hydrothermal or pneumatolitic veins in granitic X-ray data only (Demartin et al. 2000). rocks, in association with fluorine-rich minerals, such

Thortveitite

The electron-microprobe data for thortveitite from both Cuasso al Monte and Baveno (Table 6) confirm the results obtained from X-ray diffraction, the analyzed specimens being reasonably pure. Instead, the EDS spectrum of sample #2 from Cuasso al Monte shows the presence of non-negligible amounts of Y, the REE, and Zr; unfortunately, no suitable fragments were available for a quantitative chemical analysis. In view of the extreme scarcity of the thortveitite from Baveno, as well as of the very small dimensions and powdery nature of the samples, an electron-microprobe analysis could not be performed on the same specimens from which X-ray data were obtained. How- ever, the almost identical EDS spectra provided by a series of different samples of the blue variety suggest that our X-ray data should indeed correspond to the chemical composition indicated in Table 6. Further evidence in this respect is provided by the constancy of the unit-cell parameters of all the samples of the blue variety here examined, in view of the evident relationships between such parameters and the Y (and REE) content. Since the main substituents of Sc in thortveitite are generally Y and the REE, on the whole the poorest samples in these elements should be the richest in Sc. In the analyzed specimen from Baveno and in the sample from Kola studied by Voloshin et al. (1991), the Y content is the lowest on record; owing to replacement by notable amounts of Ca and especially of Mn, the Sc content for the former is appreciably lower than that for the latter. Since Mn3+ is an intense absorber (Rossman 1988), as in a number of deep blue or violet Mn-bearing minerals such as purpurite, piemontite or “violane”, the peculiar blue color of the unusual variety of thortveitite from Baveno could be ascribed to the presence of this ion, thereby providing a possible clue to its origin (Gramaccioli et al. 1999b; see also below). For the specimen from Cuasso al Monte, notable amounts of Fe substitute for Sc (up to nearly 3 wt% Fe2O3).

DISCUSSION

A general picture concerning the formation of scandium-rich minerals and their geological significance has not been established yet, owing to the scarcity of reliable physicalÐchemical data and of natural examples.

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as zinnwaldite, fluorite, and topaz [see, for instance, host notable quantities of other elements such as Y, Zr, Phan (1963), and references therein]; the miarolitic cavi- and the REE (Table 6) without extensive rearrangement. ties at Baveno and Cuasso al Monte are related to this This low selectivity is further confirmed by the exist- kind of occurrence. It should be remarked that Sc, to- ence of well-known synthetic and natural REE- or Zr- gether with other elements that commonly occur in these dominant equivalents of this mineral (Smolin & veins, especially Zr, Be, Al, Fe and the REE, forms very Shepelev 1970, Voloshin et al. 1983, 1985, Roelofsen- stable fluoride complexes. Ahl & Peterson 1989). For this reason, the relative pu- In order to account for the origin of specific Sc min- rity of some samples of thortveitite cannot be entirely erals, since scandium is rare, and as its properties are ascribed to crystal-chemical reasons, and must reflect similar to those of Y and the REE, a process of very the importance of the chemistry of the depositing solu- selective enrichment of this element is needed, and the tions. role of formation and disruption of such complexes may Since the same process leading to enrichment of Sc be essential (Gramaccioli et al. 1999 a, b, 2000). in minerals necessarily leads to its depletion in the de- Here, some thermodynamic data are available. For positing solutions, with consequent relative enrichment instance, the first stability constants, K = [MeF(nÐ1)+]/ of Zr, Y or of the REE, this accounts for the existence in [Men+][FÐ], of such complexes (as a decimal logarithm, the same locality of pure and impure varieties of at room temperature) range from 9.80 (Zr) to 7.65 (Th), thortveitite formed at various stages. If other more se- 7.10 (Al), 7.08 (Sc), 6.04 (Fe3+), 5.89 (Be), 5.76 (Mn3+), lective Sc minerals are being formed, the same process 4.85 (Sn), 4.81 (Y), 4.48 (Yb), 3.56 (La), etc.; for the leads to the eventual deposition of specific REE miner- subsequent stability constants concerning complexes als, such as complex REE oxides, gadolinite, or even (nÐ2)+ (nÐ1)+ Ð richer in fluorine, K = [MeF2 ]/[MeF ][F ], etc. zircon, of which a Sc-rich variety occurs at Baveno the trend is similar, i.e., those of Zr being the most (Gramaccioli et al. 1998). stable, followed by those of Th, etc.; an exception to On these grounds, in the presence of a sufficient this trend is the couple AlÐSc, where the stability com- concentration (activity) of fluorides or other ligands in monly is reversed (Lourié 1975), thereby enhancing the the depositing solutions, we might expect that the possibility of complexing scandium. chemical behavior of Sc and of the other elements In view of the great differences in the stability of should more or less follow the range of the stability such complexes, in the presence of adequate concentra- constants of the corresponding complexes. It is interest- tions of the fluoride ion in the depositing solutions, even ing to note that in the range of stability constants shown if the concentrations of all these elements in the solu- above, the elements ranging from Al to Be are just those tions were comparable, the values of the activity of the occurring (besides Si) in bazzite, and at least many of corresponding free ions could easily differ by several those ranging from Sc (Zr) to Yb are the most signifi- orders of magnitude. Accordingly, the chemical behav- cant components of thortveitite or of the others Sc-rich ior of scandium in these conditions should notably dif- species from Baveno. fer from that of yttrium and the REE on one hand and However, in spite of the stability of its complexes, from that of zirconium on the other (Gramaccioli et al. aluminum does not seem to follow the other elements; 1999a, b, 2000). it always remains a minor component of these scan- If the activity of the fluoride ion in a solution is re- dium-rich minerals, although it is difficult to believe that duced, a dramatic increase in the activity of the “free” it is not available in the depositing solutions. Whereas cations necessarily follows, owing to the disruption of the selectivity of the crystal structure can account for the fluoride complexes. For instance, such a case can most instances, bazzite in particular should not be too occur as a consequence of the deposition of fluorine- selective, its Al-dominant counterpart being beryl. rich minerals (such as fluorite or micas such as zinnwald- The remarkably limited content of Al in bazzite was ite, etc., which are abundant in the miarolitic cavities at taken as an indicator of partial miscibility occurring Baveno and Cuasso al Monte); this deposition may re- between this mineral and beryl (Hänni 1980, Weibel et sult from variations of temperature (or pressure), or may al. 1990, Della Ventura et al. 2000); such a conclusion also be due to mixing with an externally derived fluid is in agreement with early observations by Frondel & phase. Ito (1968), who found that substitution of Fe, Cr, etc. On examining the present results obtained for for Sc, but not for Al, can be extensive but does not Cuasso al Monte and Baveno, evidence in favor of such extend to the Sc-free end-member composition. Al- a process is available. First of all, in spite of the chemi- though the existence of a miscibility gap would not con- cal similarity among Sc, Y and the REE, bazzite, tradict our theory concerning the formation of Sc cascandite, jervisite, and scandiobabingtonite are always minerals, the presently available data are not, in our pure (Mellini & Merlino 1982, Mellini et al. 1982, opinion, sufficient to draw a definitive conclusion, and Orlandi et al. 1998), and thortveitite is commonly very other possibilities should be considered. pure. For instance, at thermodynamic equilibrium or near Whereas the structures of the other Sc minerals ap- equilibrium, the activity of the components in bazzite pear to be selective, that of thortveitite in particular can and in the depositing solutions should be strictly con-

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nected with the activity of the corresponding chemical of other scandium-rich minerals; moreover, this is an- species in the accompanying minerals. Among these other reason (apart from the values of the ionic radii) minerals, feldspars are always in close association, and for explaining the presence of notable amounts of Fe locally are intergrown with bazzite, as in the Norwe- (instead of Al) in all natural specimens of bazzite gian occurrence (Juve & Bergst¿l 1990). (Table 5). A possible equilibrium between beryl (here consid- The highest Fe content (up to nearly 7 wt% Fe2O3) ered as a component of a certain mineral of its group) observed in bazzite might be interpreted as correspond- and an alkali feldspar can be expressed as follows: ing approximately to the limit of solubility of this element in the mineral. However, in view of the + + 3+ 3+ Be3Al2Si6O18 + 2K + 4 H → 2KAlSi3O8 isomorphism between Sc and Fe in octahedral coor- 2+ + 3Be + 2H2O dination, it is difficult to imagine that there are limits for such a substitution. In any case, Frondel & Ito (1968) If the activity of KAlSi3O8 is considered to be constant, synthesized a series of Sc-doped compositions, but they 3+ and the solution is dilute, then: could not obtain the pure end-member Be3Fe 2Si6O18. The synthetic bazzite obtained by these authors contains K = exp(Ð⌬G°/RT ) = [aBe2+]3/([aK+]2[aH+]4 a maximum of 1/3 Fe (atomic), although they argued 3+ [␥.x Be3Al2Si6O18]) that the series “may extend to about 50% Fe or some- what over”. On the contrary, this end-member was ac- where a represents the activities of the corresponding tually synthesized by Hänni (1980) from chemical species in the depositing solution. For the beryl com- reaction of the solutions with the steel container, and a pound Be3Al2Si6O18, the product ␥.x between its activ- mineral of a similar composition, stoppaniite, has been ity coefficient and the molar fraction in the crystal has discovered recently (Ferraris et al. 1998, Della Ventura been used in lieu of the activity. On these grounds, such et al. 2000). a molar fraction depends very strongly upon the pH, and A possible explanation of the conflicting views con- it is instead (formally) independent of the Al activity in cerns the oxidation state of iron, which was apparently the depositing solution: in particular, a low value of pH not checked by Frondel & Ito in their products. If most in the depositing solution prevents the formation of an Fe is present as Fe2+ instead of Fe3+ (as seems to be the Al-rich member of the beryl group, and vice versa. case with many natural specimens) and only consider- Therefore, for a low pH, even if a non-negligible ing the requirements of charge balance, it is quite rea- concentration of the Be2+ ion were present in these so- sonable to imagine that a limit to the Mg or Fe2+ content lutions, no deposition of “common” beryl would take in beryl or bazzite should exist, thereby providing a rea- place, and if this mineral were present already, it would son for the limited Fe (and Mg) content of bazzite. This be destroyed to deposit feldspar. However, as an alter- limit may vary with temperature, or with the presence native, the deposition of another beryl-group mineral of alkali ions in the structure, which favor charge bal- containing Be3Al2Si6O18 as a minor component could ance. It should be remarked, however, that such a pro- equally occur, provided the pH and the activity of the cess cannot proceed beyond a certain limit, because a Be2+ ion are not too low, the activity of alkali ions is not high concentration of the alkali ions in the solution too high, and “free ions” of elements replacing Al are (which should be in equilibrium with the corresponding present in sufficient concentration. Such chemical pa- concentration in the crystal) shifts the above-described rameters might not differ extensively in most of the chemical equilibrium in favor of feldspar. observed localities yielding bazzite, thereby accounting Another possibility might be that the complexes of for its almost constant composition. Fe2+ are less stable than those of Fe3+, and therefore in Among these “free ions”, the most important for our reducing conditions the position of iron in the series of purposes are Sc3+ and Fe3+, which can derive from dis- stability constants is not so close to that of scandium. In sociation of their fluoride complexes and cannot enter any case, the question is still open.The very likely pres- the feldspar structure in substantial amounts. In other ence of the easily hydrolyzable Mn3+ in the blue variety words, the corresponding equilibrium reaction between of thortveitite is another point in favor of the presence Sc-substituted (or Fe-substituted) beryl: of notable amounts of fluorides in the depositing solutions (Gramaccioli et al. 1999b, 2000). + + Be3Sc2Si6O18 + 2K + 4 H → 2KScSi3O8 A paragenetic sequence of the minerals deposited in 2+ + 3Be + 2H2O the miarolitic cavities at Baveno and Cuasso al Monte has been reported by Pezzotta et al. (1999) and is in cannot proceed beyond a certain point, because the feld- substantial agreement with our present theory. Follow- spar structure can hardly accommodate scandium (or ing this sequence, at Baveno the deposition of fluoride- iron); this crystal-chemical detail involving an accom- rich minerals such as especially zinnwaldite led to panying mineral, together with the particular stability breakdown of the Be complexes, enhancing the activity of the beryl structure, which captures scandium, may of the free Be2+ ion in the solution and allowing the account for the very existence of bazzite even in absence deposition of beryllium minerals, such as bavenite,

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bertrandite, gadolinite, or gugiaite; a subsequent depo- BERGST¯L, S. & JUVE, G. (1988): Scandian ixiolite, pyrochlore sition of fluorite led to the breakdown of the most stable and bazzite in granite pegmatite in T¿rdal, Telemark, Nor- complexes, such as those of Sc, thereby allowing for way. A contribution to the mineralogy and geochemistry of the formation of bazzite first (when the concentration scandium and tin. Mineral. Petrol. 38, 229-243. of the Be ion was still reasonably high in the solution), BERNHARD, F., WALTER, F., ETTINGER, K., TAUCHER, J. & and finally of thortveitite or of the other scandium min- MEREITER, K. (1998): Pretulite, ScPO4, a new scandium erals, such as cascandite, jervisite, or scandiobabing- mineral from the Styrian and Lower Austrian lazulite oc- tonite, of which Baveno is the type locality. currences, Austria. Am. Mineral. 83, 625-630. According to the same authors, at Cuasso al Monte many late-stage minerals, including Be silicates, were BERTOLANI, M. (1948): Le terre rare nella bazzite di Baveno. deposited by late hydrothermal fluids, thereby altering Rend. Soc. Mineral. It. 5, 73-78. the sequence observed at Baveno. It should be remarked that these beryllium minerals at Cuasso al Monte are BIANCHI, R., PILATI, T., DIELLA, V., GRAMACCIOLI, C.M. & not exactly the same found at Baveno: for instance, MANNUCCI, G. (1988): A re-examination of thortveitite. Am. Mineral. 73, 601-607. bavenite, for which Baveno is the type locality, is alto- gether absent at Cuasso al Monte. BORIANI, A., CAIRONI, V., GIOBBI ORIGONI, E. & VANNUCCI, R. (1992): The Permian intrusive rocks of Serie dei Laghi ACKNOWLEDGEMENTS (western southern Alps). Acta Vulcanologica 2 (the Marinelli Vol.), 73-86. We are grateful to the Editor, Prof. Robert F. Mar- tin, as well to Dr. A.U. Falster and an anonymous ref- CHISTYAKOVA, M.B., MOLEVA, V.A. & RAZMANOVA, Z.P. eree, for a number of substantial suggestions for (1966): Bazzite found for the first time in the USSR. Dokl. improving our manuscript; Professor Henry A. Hänni Acad. Sci. USSR, Earth Sci. Sect. 169, 158-161.

kindly provided us with a copy of his Inaugural Disser- DELLA VENTURA, G., ROSSI, P., PARODI, G.C., MOTTANA, A., tation. We are also grateful to Mr. G. Danini, the Cura- RAUDSEPP, M. & PRENCIPE, M. (2000): Stoppaniite, tor of Museo Insubrico di Scienze Naturali at Induno (Fe,Al,Mg)4(Be6Si12O36)*(H2O)2(Na,Ⅺ), a new mineral of Olona (Varese) for having permitted the use of the the beryl group from Latium (Italy). Eur. J. Mineral. 12, unique bazzite specimen from Cuasso al Monte (sample 121-127. No. MISN 1300) for our investigation; in addition, thanks go to Mr. F. Anderbegani, Mr. R. Appiani, Dr. DEMARTIN, F., GRAMACCIOLI, C.M. & PILATI, T. (2000): Struc- Paolo Biffi, Dr. Massimo Sbacchi and Mr. Emanuele ture refinement of bazzite from pegmatitic and miarolitic Sinelli for having helped in the quest for specimens, and occurrences. Can. Mineral. 38, 1419-1424.

Mr. Luigi Saibene for technical help. ______, PILATI, T., DIELLA, V. , DONZELLI, S., GENTILE, P. & GRAMACCIOLI, C.M. (1991): The chemical composition of REFERENCES xenotime from fissures and pegmatites in the Alps. Can. Mineral. 29, 69-75. ARMBRUSTER, T., LIBOWITZKY, E., DIAMOND, L., AUERNHAMMER, M., BAUERHANSL, P., HOFFMANN, C., EBERHARD, G. (1908): Über die weite Verbreitung des IRRAN, E., KURKA, A. & ROSENSTINGL, H. (1995): Crystal Scandiums auf der Erde I. Sitzb. K. Preuss. Akad. d. Wiss. chemistry and optics of bazzite from Furkabasistunnel 38, 85. (Switzerland). Mineral. Petrol. 52, 113-126. ______(1910): Über die weite Verbreitung des Scandiums ARTINI, E. (1915): Due minerali di Baveno contenenti terre auf der Erde II. Sitzb. K. Preuss. Akad. d. Wiss. 22, 404. rare: weibyeïte e bazzite. Rend. Accad. Lincei 24, 313-319. EBY, G.N. (1973): Scandium geochemistry of the Oka ARTIOLI, G., RINALDI, R., STÅHL, K. & ZANAZZI, P.F. (1993): carbonatite complex, Oka, Quebec. Am. Mineral. 58, 819- Structure refinement of beryl by single-crystal neutron and 825. X-ray diffraction. Am. Mineral. 78, 762-768. EVANS, H.T., JR. & MROSE, M.E. (1968): Crystal chemical AURISICCHIO, C., FIORAVANTI, G., GRUBESSI, O. & ZANAZZI, studies of cesium beryl. Geol. Soc. Am., Spec. Pap. 101, 63 P.F. (1988): Reappraisal of the crystal chemistry of beryl. (abstr.). Am. Mineral. 73, 826-837. FERRARIS, G., PRENCIPE, M. & ROSSI, P. (1998): Stoppaniite, a BAKOS, F., DEL MORO, A. & VISONË, D. (1990): The Hercynian new member of the beryl group: crystal structure and crys- volcano-plutonic association of Ganna (Lake Lugano, cen- tal-chemical implications. Eur. J. Mineral. 10, 491-496. tral southern Alps, Italy). Eur. J. Mineral. 2, 373-383. FOORD, E.E., BIRMINGHAM, S.D., DEMARTIN, F., PILATI, T., BERGERHOFF, G. & NOWACKI, W. (1955): Über die GRAMACCIOLI, C.M. & LICHTE, F.E. (1993): Thortveitite Kristallstruktur des Bazzit und ihre Beziehungen zu der des and associated Sc-bearing minerals from Ravalli County, Beryll. Schweiz. Mineral. Petrogr. Mitt. 35, 410-421. Montana. Can. Mineral. 31, 337-346.

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FRONDEL, C. & ITO, J. (1968): Synthesis of the scandium ana- MOROSIN, B. (1972): Structure and thermal expansion of beryl. logue of beryl. Am. Mineral. 53, 943-953. Acta Crystallogr. B28, 1899-1903.

GRAMACCIOLI, C.M., DIELLA, V. & DEMARTIN, F. (1999a): The ORLANDI, P. (1990): Zibaldone di mineralogia italiana. Riv. role of fluoride complexes in REE geochemistry and the Mineral. It. 14, 137-144. importance of 4f electrons: some examples in minerals. Eur. J. Mineral. 11, 983-992. ______, PASERO, M. & VEZZALINI, G. (1998): Scandioba- bingtonite, a new mineral from the Baveno pegmatite, Pied- ______, ______& ______(1999b): An example of the mont, Italy. Am. Mineral. 83, 1330-1334. role of complexes in the geochemistry of transition elements in pegmatites: the formation of scandium minerals. PEYRONEL, G. (1956): The crystal structure of Baveno bazzite. In The Eugene E. Foord Memorial Symposium on NYF- Acta Crystallogr. 9, 181-186. Type Pegmatites (Denver). Can. Mineral. 37, 807-808 (abstr.). PEZZOTTA, F., DIELLA, V. & GUASTONI, A. (1999): Chemical and paragenetic data on gadolinite-group minerals from ______, ______& ______(2000): The formation of Baveno and Cuasso al Monte, southern Alps, Italy. Am. scandium minerals as an example of the role of complexes Mineral. 84, 782-789. in the geochemistry of rare earths and HFSE elements. Eur. J. Mineral. 12, 795-808. PHAN, K.D. (1963): Étude: Le scandium. Chron. Recherche Géol. Minière 324, 349-374. ______, ORLANDI, P. & CAMPOSTRINI, I. (1998): Baveno in Oberitalien: ein aussergewöhnlicher Fundort seltener Scan- RAADE, G. & ERAMBERT, M. (1999): An intergrowth of dium-Mineralien. Lapis 23(9), 27-34. scandiobabingtonite and cascandite from the Heftetjern granite pegmatite, Norway. Neues Jahrb. Mineral. HÄNNI, H.A. (1980): Mineralogische und mineralchemische Monatsh., 545-550. Untersuchungen an Beryll aus alpinen Zerrkluften. Inau- gural dissertation, Univ. of Basel, Basel, Switzerland. ROELOFSEN-AHL, J.N. & PETERSON, R.C. (1989): Gittinsite: a modification of the thortveitite structure. Can. Mineral. 27, HUTTENLOCHER, H., HÜGI, T. & NOWACKI, W. (1954): 703-708. Röntgenographische und spektrographische Untersuchungen an Bazzit. Schweiz. Mineral. Petrogr. ROSSMAN, G.R. (1988): Optical spectroscopy. In Spectroscopic Mitt. 34, 501-504. Methods in Mineralogy and Geology (F.C. Hawthorne, ed.). Rev. Mineral. 18, 207-254. JUVE, G. & BERGST¯L, S. (1990): Caesian bazzite in granite pegmatite in T¿rdal, Telemark, Norway. Mineral. Petrol. SMOLIN, YU.I. & SHEPELEV, YU.F. (1970): The crystal struc- 43, 131-136. tures of the rare earth pyrosilicates. Acta Crystallogr. B26, 484-492. LIFEROVICH, R.P., SUBBOTIN, V.V., PAKHOMOVSKY, YA.A. & LYALINA, M.F. (1998): A new type of scandium minerali- VOLOSHIN, A.V., GORDIYENKO, V.V. & PAKHOMOVSKY, YA.A. zation in phoscorites and carbonatites of the Kovdor Mas- (1991): Scandium mineralization and the first find of sif, Russia. Can. Mineral. 36, 971-980. thortveitite Sc2Si2O7, in granite pegmatite on the Kola Pe- ninsula. Dokl. Akad. Nauk 318, 972-976 (in Russ.). ______, YAKOVENCHUK, V.N., PAKHOMOVSKY, YA.A., BOGDANOVA, A.N. & BRITVIN, S.N. (1997): Juonniite Ð a ______, PAKHOMOVSKY, YA.A. & TYUSHEVA, F.N. (1983): new mineral of scandium from dolomite carbonatites of the Keiviite, Yb2Si2O7, a new ytterbium silicate from Kovdor massif. Zap. Vser. Mineral. Obshchest. 126(4), 80- amazonite pegmatites of the Kola Peninsula. Mineral. Zh. 88 (in Russ.). 5(5), 94-99 (in Russ.).

LOURIÉ, Y. (1975): Aide-mémoire de chimie analytique. MIR, ______, ______& ______(1985): Keiviite-(Y) Ð a new Moscow, Russia. yttrium diorthosilicate, and thalénite from amazonite pegmatites of the Kola Peninsula. Diortho- and MANNUCCI, G., DIELLA, V., GRAMACCIOLI, C.M. & PILATI, T. triorthosilicates of yttrium. Mineral. Zh. 7(6), 79-94 (in (1986): A comparative study of some pegmatitic and fis- Russ.). sure monazite from the Alps. Can. Mineral. 24, 469-474. WEIBEL, M., GRAESER, S., OBERHOLZER, W.H., STALDER, H.A. MELLINI, M. & MERLINO, S. (1982): The crystal structure of & GABRIEL, W. (1990): Die Mineralien der Schweiz (5th cascandite, CaScSi3O8(OH). Am. Mineral. 67, 604-609. ed.). Birkhäuser, Basel, Switzerland.

______, ______, ORLANDI, P. & RINALDI, R. (1982): Cascandite and jervisite, two new scandium silicates from Received February 9, 2000, revised manuscript accepted No- Baveno, Italy. Am. Mineral. 67, 599-603. vember 3, 2000.

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